Satis configuration file description

Better upfront description of what a Satis configuration file actually is. Was previously not clear the name didn't matter until further down.
Further simplified Satis Config intro
Update SPDX license identifiers
Fix test suite
Allow for "proprietary" as license identifier
optimizing loops
removing incorrect optimization
Merge pull request #1615 from shanethehat/loop-optimization

Optimizing loops
Specific schema validation failure messages (fixes issue #1616)
Add other valid versions to test for stability
adding use statement
Merge pull request #1618 from ChristianRiesen/master

Small enhancement of test to test more stable version strings
Merge pull request #1617 from shanethehat/schema-validation-messages

Specific schema validation failure messages (fixes issue #1616)
Merge pull request #1609 from jeffturcotte/patch-1

Satis configuration file description
Show proper error message when a git bitbucket repo is loaded as hg repo, refs composer/packagist#294
Update the way github authorization is handled, fixes #1632

Since api.github.com redirects to s3 for downloads and s3 does not like Authorization
headers, we have to rely on the access_token query param. Otherwise php follows redirects
but still sends the Authorization header to all following requests.
Add lib-ICU platform package
Fixed the "access_token query param" (5b1f314) fix
Extract archives into temp dir to shorten paths and avoid issues on windows, fixes #1591
Only downgrade providers but not the notification url